Persist Markee authorization nonces for production

[Corantin]

Goal

Replace the in-memory Markee opt-in and revenue-claim authorization challenge maps with a shared, atomic, single-use nonce store suitable for production serverless deployments.

Scope

• Persist challenges for /api/markee/authorize and /api/markee/claim/authorize.
• Preserve the five-minute TTL and consume-before-verification replay protection.
• Make nonce consumption atomic across instances.
• Re-enable challenge issuance in production only after the durable store is configured.
• Add multi-instance/replay and expiry tests.
• Document and provision the required Vercel environment variables.

Current behavior

Both endpoints intentionally fail closed in production because an in-memory Map cannot prevent replay across cold starts or concurrent serverless instances.

Tracked from PR #951 review.